1. Field of the Invention
This invention relates generally to an interactive terminal system wherein information is transferred between a communications controller and a number of work stations over a single line, and relates more particularly to the prepolling of a work station prior to the communications controller transferring a block of information to the work station.
2. Description of the Prior Art
An interactive terminal system includes a central system which is operative with a number of work stations. Very often the economics of the system dictates that the work stations share a common bus rather than connecting each work station to the central system directly.
The shared bus is feasible when the average rate of data transfer does not exceed the capacity of the system.
Of course, two terminals sharing the common bus may not simultaneously communicate with the central system, so it is customary for the central system to interrogate each terminal in turn. Each work station is assigned a distinct address. The central system may query a work station by sending its address and a command character asking if the work station has information to send to the central system; or the central system may follow the address by a command character indicating that the central system is transferring information to the addressed work station.
The central system may store a polling list establishing the sequence in which to polled the terminals. Certain terminals may have their addresses on the polling list more than once so that they may be polled more often. Polling systems are described in "Introduction to Teleprocessing" by James Martin and published by Prentice-Hall, Inc. of Englewood Cliffs, N.J. in 1972.
However, as the data transfer rates between the central systme and the work stations increase, and the work stations themselves are being operated off-line, the above polling techniques are not adequate for maximum productivity. The data transfer rate of 750,000 bits per second requires that a work station be conditional to receive the information block immediately upon receiving its address. Since the work station is normally operating in a background mode, an inordinate amount of hardware is required for the work station to switch over to the receive mode and receive the block of information.